The present invention relates to the preparation of metal alkyls useful in the preparation of compound semi-conductor materials.

Compound semiconductor materials, eg materials such as gallium arsenide, indium phosphide, gallium phosphide and cadmium mercury telluride, are well known materials having uses in the electronics industry in such applications as microwave oscillators, semiconductor light emitting diodes and lasers, and infrared detectors.

Such materials have been made in the past by forming, usually on a substrate crystal, one or more active layers, by the method of vapour phase epitaxy (VPE).

It has been known for some time to produce by VPE compound semiconductors of the form M^AQ^A where M^A is Group III element and Q^A is Group V element by reacting a trialkyl of the element M^A with a gaseous compound, eg a hydride, of the Group V element Q^A. This method is a suitable method of preparing gallium arsenide from Ga(CH₃)₃ and AsH₃ for example.

Consequently, trialkyl gallium compounds, in particular trimethyl gallium, have become important in the production of semiconductor materials.

« It is well known to those skilled in the art that the presence of impurities in semiconductor materials has a profound effect on the electrical and other properties of the materials. In order to control the properties it is therefore desirable to produce such materials in a high purity form. This means that the precursor materials such as trialkyl gallium compounds used in the manufacture of the semiconductor materials are desirably as pure as possible.

The reaction between gallium trichloride and a Grignard reagent containing an alkyl radical is known but has not hitherto been used for the preparation of trialkyl gallium compounds. This is because the reaction has been carried out in diethyl ether as solvent and has resulted in the formation of trialkyl gallium - diethyl ether adducts. Such adducts cannot be broken down directly into the trialkyl gallium compound without severe contamination from the ether.

According to the present invention a method of producing a trialkyl gallium compound (R_A)₃Ga, where R is selected from methyl and ethyl, includes Step A which is the step of reacting a gallium trihalide with a Grignard reagent of formula R_A Hg Q, where Q is a halogen, the reaction being carried out in the presence.of an ether of the formula R₁R₂0 having a boiling point at least 50 Celsius degrees above that of the trialkyl gallium compound _(RA)3Ga, wherein _R1 and R₂ are organic radicals at least one of which has at least 5 carbon atoms.

The organic radicals R₁ and R₂ may be aromatic and/or aliphatic radicals. They are preferably alkyl or phenyl radicals. Preferably R₁R₂O is an ether having a boiling point more than 100C^o above that of the trialkyl gallium compound. The radicals R₁ and R₂ may each independently have from 1 to 14 carbon atoms but preferably together R₁ and R₂ have between seven and twelve carbon atoms inclusive.

Desirably, R₁R₂O is di-isopentyl ether (3,3-dimethylpropyl) ether. Other examples of R₁R₂O are diphenyl ether and anisole (methylphenyl ether). Preferably, Q is iodine and the gallium trihalide is a chloride.

The main product of Step A used in the method according to the present invention is an adduct of the trialkyl gallium (R_A)₃G^a and the ether R₁R₂0. However, in contrast to the known Grignard reaction mentioned above, the reaction of Step A provides as a product an adduct which may be converted directly into the radical-free trialkyl gallium in a high purity form in a single step. Furthermore, this conversion may be carried out directly using the product of the Grignard reaction in its production vessel without further separation from the contents of the vessel, eg the reagents used in its formation.

The adduct may be converted into the radical-free trialkyl gallium compound by Step B as follows:

• heating the adduct (R_A)₃Ga.OR₁R₂ as a vapour,and separating the trialkyl gallium compound by fractional distillation of its vapour.

Preferably, Step B is preceded by the removal of volatile impurities by evacuation or distillation at a temperature below the decomposition temperature of the adduct (R_A)₃Ga. ^R₁^R₂⁰.

In Step A specified above the Grignard reaction may be carried out at room temperature (20 C), or preferably at an elevated temperature eg 70-110⁰C. In the latter case the elevated temperature used serves to remove volatile impurities prior to dissociation of the adduct in Step B.

An example of a method embodying the present invention will now be described in detail.

Trimethyl gallium in pure form was produced by the following method.

Di-isopentyl ether was first dridd over sodium wire and then redistilled.

Methyl magnesium iodide was then prepared in the ether as follows.

Methyl iodide (27 ml, 0.43mol) was added to a suspension of magnesium turnings (10.2 g, 0.42 mol) in the di-isopentyl ether (200 ml). The rate of addition of methyl iodide was such that the mixture remained very hot throughout the addition and did not require external heating.

The following reaction was then carried out in an atmosphere of dry nitrogen.

The solution of methyl magnesium iodide in di-isopentyl ether was added over a period of one hour to a stirred solution of anhydrous gallium trichloride (25 g, 0.14 mol) in di-isopentyl ether (50 ml). The rate of addition of the Grignard solution was such that the reaction mixture stayed hot throughout the addition and did not require external heating.

After complete addition of the Grignard solution the reaction mixture was heated by an oil bath at a bath temperature of 80-100°C for 18 hr after which time the mixture had become pure white.

Pure trimethylgallium (boiling point 56°C) was obtained by distillation through a 30 cm x 1.5 cm column packed with glass beads or Fenske helices. The bath temperature during distillation was higher than 180°C (~ 200°C). Only one fraction was collected.

The yield of (CH₃)₃Ga was 11.0 g (67.5% based on the amount of gallium trichloride used).

The product structure was checked by mass spectrometry using a VG Micromass 13 mass spectrometer using the batch inlet, and the results are given in Table 1 as follows.

The data in Table 1 confirms that the product was trimethyl gallium.

Trimethyl gallium may be prepared by a method analogous to the above example but using anisole (preferably at a reaction temperature of about 90-100°C) or alternatively diphenyl ether (preferably at a reaction temperature of about 80-90°C) as solvent in the Grignard reagent/gallium trichloride reaction. In these alternatives distillation of trimethyl gallium from the intermediate adducts is carried out using oil bath temperatures of about 150⁰C (for the anisole adduct) and 140°C (for the diphenyl ether adduct) respectively.

Trimethyl gallium samples produced by the method of the above example have proved suitable for use in growing high quality epitaxial layers of GaAs (from the reaction of Ga(CH₃)₃ and AsH₃) with carrier concentrations of from 3 to 4.5 x 10¹⁵ cm^-3 at 298K which are better quality layers than those produced from commercially available trimethyl gallium.